An IP Multimedia Subsystem (IMS) is defined by 3GPP as a mobile network infrastructure that enables convergence of voice, video, data, and mobile network technology over an Internet Protocol (IP) based infrastructure. It fills the gap between cellular and internet technologies by delivering multimedia to mobile users via an IP connection.
FIG. 1 illustrates a common implementation of an IMS infrastructure 100. The IMS infrastructure 100 delivers multimedia services to users of various mobile devices 102 via an IP network 104. In particular, the IMS infrastructure 100 comprises telecommunications carrier systems 106 having application servers 108 that host various multimedia services, a billing component 110 that charges for the use of the multimedia services, and a core IMS system 112 that establishes a transfer of desired multimedia services to the requesting mobile device or devices 102. The mobile devices 102 establish IP communication access with the telecommunications carrier systems 106 via various network access points 114, such as cellular towers of a mobile telecommunication network. The access points 114 are coupled to the telecommunications carrier systems via a public or private network 104.
The telecommunications carrier provides mobile telecommunications service to the mobile devices 102. The mobile devices 102 may be laptops, mobile phones, PDAs, media players, and similar devices that are able to connect to the access points 114 and transmit data using a variety of transmission standards, such as GPRS (General Packet Radio Service), EDGE (Enhanced Data Rates for GSM Evolution), UMTS (Universal Mobile Telecommunications System), and W-CDMA (Wideband Code Division Multiple Access). The network 104 which is used to transfer IP packets from the access points 114 to the telecommunications carrier systems 106, can be owned and operated by the telecommunications carrier or by a third party service provider.
The application servers 108 provide an integrated and standardized network platform to allow the telecommunications carrier to offer a variety of multimedia services to the mobile devices 102. The application servers 108 are not only responsible for hosting and executing the multimedia services, but also provide the interface against the core IMS server using a SIP protocol. A single application server may host multiple services, for example, telephony and messaging services can run on one application server, and a single service may require multiple application servers.
The application servers 108 can take the form of presence servers, list management servers, and/or instant messaging servers:                A presence server provides services to collect, manage, and distribute real-time availability of the mobile devices 102 and a means for communicating among them. It allows users of the mobile devices to both publish their presence information and subscribe to the service in order to receive notification of changes by other users.        A list management server creates and manages network-based group definitions and associated lists of members for defined groups. A list management server can maintain access lists, permissions, and other service-specific properties associated with groups and group members. Use of a list management server permits a user's contact list, such as an email list of a personal address book, to be specified and used in an application independent manner. This allows a user's contact list to be used by the mobile device 102 of the user. It is also used to provide buddy lists for instant messaging or other services.        An instant messaging server provides a communication service that allows mobile devices 102 to send and receive messages instantly. Mobile device users are able to deliver messages containing rich text, images, audio, video, or the combination of these over the IP network 104.        
The core IMS system 112 is used to establish and control the multimedia session among the mobile devices 102. The core IMS system 112 includes a CSCF (Call Session Control Function) component 116, such as SIP servers or proxies, and a HSS (Home Subscriber Server) component 118. The CSCF 116 establishes, monitors, supports, and releases multimedia sessions and manages the user's interactions with the application servers 108. The CSCF 116 can serve (S-CSCF), proxy (P-CSCF), or interrogate (I-CSCF) multimedia sessions. In functioning as a proxy server (P-CSCF), the CSCF 116 can control the multimedia session. The CSCF 116 handles SIP registration of the mobile devices 102 and processes SIP signal messaging of the appropriate application server to invoke the services requested by a mobile device.
The HSS 118 can be accessed by the CSCF 116 using Diameter or other AAA (Authentication, Accounting, and Authorization) protocols. The HSS 118 is a master database that supports the CSCF 116 in handling the multimedia sessions between mobile devices 102. The HSS 118 stores unique mobile device identities, such as IMPI (IP Multimedia Private ID), IMPU (IP Multimedia Public Identity), as well as hardware identities (e.g., IMEI, MAC, Serial Numbers, etc.) and performs authentication and authorization of the mobile devices. The HSS 118 provides the CSCF 116 with the device identity used to establish multimedia sessions with the mobile devices.
The billing component 110 is connected to the core IMS system 112 via an Open Services Access-Gateway (OSA-GW) or directly through data service components over IP protocols. When the mobile device 102 implements a multimedia session, the billing component 110 is invoked and the user of the mobile device 102 is charged. The charge may be quantified in terms of download quantity (i.e., size of data transfer) or duration (i.e., length of data transfer).
When a multimedia session is to be established as either a download to a mobile device or as shared multimedia between multiple mobile devices 102, it is necessary for the telecommunications carrier to allocate bandwidth for IP transmission of the desired multimedia content. Such allocation of bandwidth limits the available bandwidth of the telecommunications carrier and increases network congestion. Thus, it is desirable to implement a system and method which would free telecommunications carriers from allocating substantial bandwidth towards multimedia content, while maintaining control over multimedia content billing.